[Duffman]

Well it is both true and false.

Diesel particulate filters require fuel to function so all diesels this year burn more fuel than last year to be cleaner. Fighting NOx is costing us MPG as well. But you are right that we are getting cleaner every year and FE is holding for the most part. Like I said earlier though, most of the basic tech and theory was available a long time ago, its largely more implemented now though because we have better manufacturing tools to implement it.

edit: I forgot to add as well, changes in fuel formulation have also had a big effect on the cleanliness of cars, and that’s really an external to the idea you are putting forward.